Typically, a solid-state laser cavity contains a crystal host material that is doped with a small amount of an activator ion. This ion can be pumped by a light source such as a flash lamp or more commonly, a diode laser of suitable frequency. The light from the pump is absorbed by the gain medium, i.e., the doped host material, creating a population inversion that causes stimulated emission of coherent light. The laser cavity generally contains mirrors at each end that reflect most of the emitted light back into the cavity, which stimulates increasingly greater emission. This emitted light is coherent, meaning that the light is a single wavelength, is single phase and unidirectional. The output light can be in the form of continuous or pulsed emission.
While the gain medium can be the only crystal regime of a laser cavity, solid-state lasers often employ several crystal regimes that serve a series of purposes including thermal management, mechanical strength, waveguiding, and the like. These regimes can occur in the form of a series of layers that have similar dimensions, but with different chemical compositions where the different compositions reflect the different functions. The compositions of the different regimes generally include a common host material with one or more different dopants (and/or different dopant concentrations) in different regimes, with the amount and type of dopant(s) selected based upon the function of each regime.
One such regime can minimize amplified spontaneous emission (ASE). In ASE, photons at the lasing wavelength are occasionally and spontaneously emitted from the lasing ions with population inversions. In contrast to stimulated emissions (lasing), in which the emission is coherent and directional, the ASE photons emit in all directions. These emissions can reflect back from the crystal surface and induce further emission from the excited states thus depleting the population inversion and reducing the power and efficiency of the laser cavity. To minimize ASE, a regime can be included in the laser cavity that is doped with an ion that can absorb the spontaneously emitted photons and allow them to relax as thermal emission, thus preventing them from inducing further depletion of the population inversion of the lasing ions. Typically an ASE regime is situated around the edges of the lasing crystal as cladding.
Laser waveguides also use multiple different regimes. Waveguides include two or more regimes with similar lattice structures whereby an internal portion, or core, contains a material having an index of refraction that is larger than that of an outer portion material, or cladding. Thus, total internal reflectance can be achieved and essentially all light that enters will exit downstream. This condition is most commonly exploited in fiber optics but can also work for other applications. In a planar waveguide, a third layer can be applied to form a three layer construct. If conditions are selected to ensure that that index of refraction of the central layer is larger than the outer regimes, total internal reflection can be achieved and a planar waveguide created.
Whatever the application of different regimes, the interface between adjacent regimes is of importance as it can have a large effect on overall beam quality. For instance, if the output beam is to be frequency manipulated through a non-linear process (for example second harmonic generation or optical parametric oscillation), it is important to have interfaces between adjacent regimes with controlled lattice orientations to control polarization interaction with the pump light.
Various methods have been devised to form multi-regime laser cavities. One method includes direct bonding of the different regime materials to one another. Use of glues, fluxes or other bonding materials has been examined but can be unacceptable due to degradation of the optical beam quality. Other direct bonding methods include pressure bonding, electrical potential fusion and diffusion bonding,
A second method for the production of a multi-regime laser cavity is the direct growth of different regimes in layers on an underlying substrate (e.g., a previously formed regime). Direct growth has been accomplished through epitaxial growth in which one regime material acts as a substrate and a second regime material is deposited on the surface in a stepwise controlled manner. The grown layer can in some formation methods adopt the general structural characteristics of the substrate (such as same lattice type and/or similar dimensions).
Liquid phase epitaxy (LPE) has been used to form multiple regimes. LPE employs high temperature fluxes to dissolve the substrate material and deposit the new layers on the substrate seed via supersaturation. It typically employs molten salts that are usually mixtures of lead oxide and boron oxide or other metal oxides that melt between 1200° C. and 1600° C. and impart modest solubility to the desired layer material.
Hydrothermal methods have also been described in formation of single monolithic crystals with different regimes. Hydrothermal techniques, in which a temperature differential is developed across a reactor to create a supersaturated solution leading to crystal growth on a seed, have been utilized for bulk single crystal growth.
Unfortunately, formation methods have been limited to relatively simplistic overall designs, with the most complicated geometries including successive layers grown on or adhered to one another, optionally followed by cutting and polishing the resulting bulk crystals to form the final product. What are needed in the art are heterogeneous crystals having more complicated geometries, which can be used for example to improve design of the geometry of a laser cavity and thus better control the application of each regime for more efficient use.